Polar Coronal Holes During Solar Cycles 22 and 23

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 (from 1990 to end of 2003). In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes (PCHs) appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH.     

Polar Coronal Holes During Cycles 22 and 23

The National Solar Observatory/Kitt Peak synoptic rotation maps of the magnetic field and of the equivalent width of the He i 1083 nm line are used to identify and measure polar coronal holes from September 1989 to the present. This period covers the entire lifetime of the northern and southern polar holes present during cycles 22 and 23 and includes the disappearance of the previous southern polar coronal hole in 1990 and and formation of the new northern polar hole in 2001. From this sample of polar hole observations, we found that polar coronal holes evolve from high-latitude (60° ) isolated holes. The isolated pre-polar holes form in the follower of the remnants of old active region fields just before the polar magnetic fields complete their reversal during the maximum phase of a cycle, and expand to cover the poles within 3 solar rotations after the reversal of the polar fields. During the initial 1.2–1.4 years, the polar holes are asymmetric about the pole and frequently have lobes extending into the active region latitudes. During this period, the area and magnetic flux of the polar holes increase rapidly. The surface areas, and in one case the net magnetic flux, reach an initial brief maximum within a few months. Following this initial phase, the areas (and in one case magnetic flux) decrease and then increase more slowly reaching their maxima during the cycle minimum. Over much of the lifetime of the measured polar holes, the area of the southern polar hole was smaller than the northern hole and had a significantly higher magnetic flux density. Both polar holes had essentially the same amount of magnetic flux at the time of cycle minimum. The decline in area and magnetic flux begins with the first new cycle regions with the holes disappearing about 1.1–1.8 years before the polar fields complete their reversal. The lifetime of the two polar coronal holes observed in their entirety during cycles 22 and 23 was 8.7 years for the northern polar hole and 8.3 years for the southern polar hole.     

Coronal Holes and Open Magnetic Flux over Cycles 23 and 24

As the observational signature of the footprints of solar magnetic field lines open into the heliosphere, coronal holes provide a critical measure of the structure and evolution of these lines. Using a combination of Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT), Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), and Solar Terrestrial Relations Observatory/Extreme Ultraviolet Imager (STEREO/EUVI A/B) extreme ultraviolet (EUV) observations spanning 1996?–?2015 (nearly two solar cycles), coronal holes are automatically detected and characterized. Coronal hole area distributions show distinct behavior in latitude, defining the domain of polar and low-latitude coronal holes. The northern and southern polar regions show a clear asymmetry, with a lag between hemispheres in the appearance and disappearance of polar coronal holes.     

Solar Wind Forecasting with Coronal Holes

An empirical model for forecasting solar wind speed related geomagnetic events is presented here. The model is based on the estimated location and size of solar coronal holes. This method differs from models that are based on photospheric magnetograms (e.g., Wang–Sheeley model) to estimate the open field line configuration. Rather than requiring the use of a full magnetic synoptic map, the method presented here can be used to forecast solar wind velocities and magnetic polarity from a single coronal hole image, along with a single magnetic full-disk image. The coronal hole parameters used in this study are estimated with Kitt Peak Vacuum Telescope He I 1083 nm spectrograms and photospheric magnetograms. Solar wind and coronal hole data for the period between May 1992 and September 2003 are investigated. The new model is found to be accurate to within 10% of observed solar wind measurements for its best 1-month period, and it has a linear correlation coefficient of ∼0.38 for the full 11 years studied. Using a single estimated coronal hole map, the model can forecast the Earth directed solar wind velocity up to 8.5 days in advance. In addition, this method can be used with any source of coronal hole area and location data.     

Direct and Indirect Thermospheric Heating Sources for Solar Cycles 21–23

Solar variability is often cast in terms of radiative emission and the associated long-term climate response; however, growing societal reliance on technology is creating more interest in day-to-day solar variability. This variability is associated with both solar radiative and solar wind emissions. In this paper we explore the combined effects of radiative and solar wind fluctuations at Earth. The fluctuations in radiative and geomagnetic power create an extended interval of solar maximum for the upper atmosphere. We use a trio of empirical models to estimate, over the last three solar cycles, the relative contributions of solar extreme ultraviolet (UV) power, Joule power, and particle kinetic power to the Earth’s upper atmosphere energy budget. Daily power values are derived from three source models. The SOLAR2000 solar irradiance specification model provides estimates of the daily extreme and far UV solar power input. Geomagnetic power is derived from a combination of satellite-estimated particle precipitation power and an empirical model of Joule power from hemispherically integrated estimates of high-latitude energy deposition. During the interval 1975 to 2003, the average daily contributions were: particles – 36 GW, Joule – 95 GW and solar – 464 GW for a total of 595 GW. Solar wind-driven geomagnetic power provided 22% of the total global upper atmospheric energy. In the top 15 power events, geomagnetic power contributed two-thirds of the total power budget. In each of these events, Joule power alone exceeded solar power. With rising activity, Joule power becomes the most variable element of solar upper atmosphere interactions.     

The Temperature of the Low Corona During Solar Cycles 21–23

Observations of the forbidden coronal lines Fe xiv 530.3 nm and Fe x 637.4 nm obtained at the National Solar Observatory at Sacramento Peak are used to determine the variation of coronal temperature at latitudes above 30^∘ during solar activity cycles 21–23. Features of the long-term variation of emission in the two lines are also discussed. Temperatures at latitudes below 30^∘ are not studied because the technique used to determine the coronal temperature is not applicable in active regions. The polar temperature varies cyclically from approximately 1.3 to 1.7 MK. The temperatures are similar in both hemispheres. The temperature near solar minimum decreases strongly from mid-latitudes to the poles. The temperature of the corona above 80^∘ latitude generally follows the sunspot cycle, with minima in 1985 and 1995–1996 (cf. 1986 and 1996 for the smoothed sunspot number, Rz) and maxima in 1989 and 2000 (cf. 1989 and 2000 for Rz). The temperature of the corona above 30^∘ latitude at solar maximum is nearly uniform, i.e., there is little latitude dependence. If the maximum temperatures of cycles 22 and 23 are aligned in time (superposed epochs), the average annual N + S temperature (average of the northern and southern hemisphere) in cycle 23 is hotter than that in cycle 22 at all times both above 80^∘ latitude and above 30^∘ latitude. The difference in the average annual N + S maximum temperature between cycles 23 and 22 was 56 kK near the poles and 64 kK for all latitudes above 30^∘. Cycle 23 was also hotter at mid-latitudes than cycle 22 by 60 kK. The last 3 years of cycle 21 were hotter than the last 3 years of cycle 22. The difference in average annual N + S temperatures at the end of cycles 21 and 22 was 32 kK near the poles and 23 kK for all latitudes above 30^∘. Cycle 21 was also hotter at mid-latitudes than cycle 22 by at least 90 kK. Thus, there does not seem to be a solar-cycle trend in the low-coronal temperature outside of active regions.     

Energy Gain of Positive Ions in Solar Polar Coronal Holes

We have investigated heating of solar polar coronal holes and acceleration of fast solar wind by means of lower hybrid (LH) waves. A three-fluid Maxwell model comprising electrons, protons, and α-particles is employed at around two solar radii heliocentric distance, where wave dissipation starts to be dominated by collisionless processes. We suggest specific wavenumber ranges corresponding to LH as well as stochastic instabilities and find that these instabilities may bring about a significant energy gain in positive ions.     

宁静日冕和冕洞研究现状

主要论述宁静日冕洞，以及日冕加热问题的研究现状。讨论了宁静日冕的理论模型、观测模型和混合模型，以及冕洞区大气模型和太阳风加热问题。最后对计划中的日冕空间探测作了简要介绍。     

Intermediate-Term Periodicities in Soft X-ray Flare Index During Solar Cycles 21, 22 and 23

We have analyzed the intermediate-term periodicities in soft X-ray flare index (FI_SXR) during solar cycles 21, 22 and 23. Power-spectral analysis of daily FI_SXR reveals a significant period of 161 days in cycle 21 which is absent during cycles 22 and 23. We have found that in cycle 22 periodicities of 74 and 83 days are in operation. A 123-day periodicity has been found to be statistically significant during part of the current solar cycle 23. The existence of these periodicities has been discussed in the light of earlier results.     

Contrast of Coronal Holes and Parameters of Associated Solar Wind Streams

It is shown that the contrast of coronal holes, just as their size, determines the velocity of the solar wind streams. Fully calibrated EIT images of the Sun have been used. About 450 measurements in 284 Å have been analyzed. The time interval under examination covers about 1500 days in the declining phase of cycle 23. All coronal holes recorded for this interval in the absence of coronal mass ejections (CMEs) have been studied. The comparison with some other parameters (e.g. density, temperature, magnetic field) was carried out. The correlations with the velocity are rather high (0.70?–?0.89), especially during the periods of moderate activity, and could be used for everyday forecast. The contrast of coronal holes is rather small.     

Resonant Heating of Ions by Parallel Propagating Alfvén Waves in Solar Coronal Holes

Resonant heating of H, O+5, and Mg+9 by parallel propagating ion cyclotron Alfven waves in solar coronal holes at a heliocentric distance is studied using the heating rate derived from the quasilinear theory. It is shown that the particle-AlfVen-wave interaction is a significant microscopic process. The temperatures of the ions are rapidly increased up to the observed order in only microseconds, which implies that simply inserting the quasilinear heating rate into the fluid/MHD energy equation to calculate the radial dependence of ion temperatures may cause errors as the time scales do not match. Different species ions are heated by Alfven waves with a power law spectrum in approximately a mass order. To heat O+5 over Mg+9 as measured by the Ultraviolet Coronagraph Spectrometer (UVCS) in the solar coronal hole at a region ≥1.9.R, the energy density of Alfven waves with a frequency close to the O+5-cyclotron frequency must be at least double of that at the Mg+9-cyclotron frequency. With an appropriate wa     

Magnetic Topology of Active Regions and Coronal Holes: Implications for Coronal Outflows and the Solar Wind

During 2??C?18 January 2008 a pair of low-latitude opposite-polarity coronal holes (CHs) were observed on the Sun with two active regions (ARs) and the heliospheric plasma sheet located between them. We use the Hinode/EUV Imaging Spectrometer (EIS) to locate AR-related outflows and measure their velocities. Solar-Terrestrial Relations Observatory (STEREO) imaging is also employed, as are the Advanced Composition Explorer (ACE) in-situ observations, to assess the resulting impacts on the solar wind (SW) properties. Magnetic-field extrapolations of the two ARs confirm that AR plasma outflows observed with EIS are co-spatial with quasi-separatrix layer locations, including the separatrix of a null point. Global potential-field source-surface modeling indicates that field lines in the vicinity of the null point extend up to the source surface, enabling a part of the EIS plasma upflows access to the SW. We find that similar upflow properties are also observed within closed-field regions that do not reach the source surface. We conclude that some of plasma upflows observed with EIS remain confined along closed coronal loops, but that a fraction of the plasma may be released into the slow SW. This suggests that ARs bordering coronal holes can contribute to the slow SW. Analyzing the in-situ data, we propose that the type of slow SW present depends on whether the AR is fully or partially enclosed by an overlying streamer.     

Short-Period Fluctuations in Coronal Mass Ejection Activity during Solar Cycle 23

We have constructed a time series of the number of coronal mass ejections (CMEs) observed by SOHO/LASCO during solar cycle 23. Using spectral analysis techniques (the maximum entropy method and wavelet analysis) we found short-period (< one year) semiperiodic activity. Among others, we found interesting periodicities at 193, 36, 28, and 25 days. We discuss the implications of such short-period activity in terms of the emergence and escape of magnetic flux from the convection zone, through the low solar atmosphere (where these periodicities have been found for numerous activity parameters), toward interplanetary space. This analysis shows that CMEs remove the magnetic flux in a quasiperiodic process in a way similar to that of magnetic flux emergence and other solar eruptive activity.     

The Solar Irradiance Spectrum at Solar Activity Minimum Between Solar Cycles 23 and 24

On 7 February 2008, the SOLAR payload was placed onboard the International Space Station. It is composed of three instruments, two spectrometers and a radiometer. The two spectrometers allow us to cover the 16?–?2900 nm spectral range. In this article, we first briefly present the instrumentation, its calibration and its performance in orbit. Second, the solar spectrum measured during the transition between Solar Cycles 23 to 24 at the time of the minimum is shown and compared with other data sets. Its accuracy is estimated as a function of wavelength and the solar atmosphere brightness-temperature is calculated and compared with those derived from two theoretical models.     

Periodic Appearance of Coronal Holes and the Related Variation of Solar Wind Parameters

We compared the variability of coronal hole (CH) areas (determined from daily GOES/SXI images) with solar wind (daily ACE data) and geomagnetic parameters for the time span 25 January 2005 until 11 September 2005 (late declining phase of solar cycle 23). Applying wavelet spectral analysis, a clear 9-day period is found in the CH time series. The GOES/SXI image sequence suggests that this periodic variation is caused by a mutual triangular distribution of CHs ∼120° apart in longitude. From solar wind parameters a 9-day periodicity was obtained as well, simultaneously with the 9-day period in the CH area time series. These findings provide strong evidence that the 9-day period in solar wind parameters, showing up as higher harmonic of the solar rotation frequency, is caused by the “periodic” longitudinal distribution of CHs on the Sun recurring for several solar rotations. The shape of the wavelet spectrum from the Dst index matches only weakly with that from the CH areas and is more similar to the wavelet spectrum of the solar wind magnetic field magnitude. The distinct 9-day period does not show up in sunspot group areas which gives further evidence that the solar wind modulation is strongly related to CH areas but not to active region complexes. The wavelet power spectra for the whole ACE data range (∼1998 – 2006) suggest that the 9-day period is not a singular phenomenon occurring only during a specific time range close to solar minimum but is occasionally also present during the maximum and decay phase of solar cycle 23. The main periods correspond to the solar rotation (27^d) as well as to the second (13.5^d) and third (9^d) harmonic. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

太阳活动22周以来的冕洞和地磁指数

本文对２２太阳活动用以来的中低纬冕洞和地磁指数Ａｐ进行了统计。对以月、季、年及２２周以来不同时段冕洞和地磁指数（Ｐｌａｎｅｔａｒｙ的Ａ指教）的时段合成图进行了分析。     

Investigation of Hemispherical Variations of Soft X-Ray Solar Flares during Solar Cycles 21 to 24

Solar System Research - In the current investigation we have studied the distribution as well as the asymmetry of solar X-ray flares during the period 1976–2017 which corresponds to solar...